Spider bites and venoms

Spider venoms are a cocktail of many chemicals.

Some are neurotoxins, which evolved to kill or immobilise arthropods like insects by attacking their nervous systems; others are cytotoxins which help break down the tissue so the spider can ingest a liquefied meal. Unfortunately, a few of these chemicals can be seriously toxic to people.

How does spider venom work in humans?

Venoms are chemicals of biological origin (i.e. made by an animal) used for the purpose of attack or defence. Venoms are made by specialised organs, such as modified salivary glands, and are delivered via specialised systems such grooved or hollow fangs. Most venoms consist of a complex mixture of chemical substances, including proteins, peptides, sugars and other substances. Venoms may affect many systems of the body. Common venom effects include paralysis, interference with blood clotting, breakdown of muscle, pain, breakdown of tissues and effects on the cardiorespiratory system (the heart and lungs).

There are basically two types of venom that have an effect on humans: neurotoxic and cytotoxic (or necrotic) venoms. Neurotoxic venoms work directly on the nervous system. The best known example is the venom of the Black Widow/Redback spiders (Latrodectus species). Necrotic venoms cause damage to the tissues, such as blisters and lesions. There are no confirmed records of spider bites in Australia causing necrotic lesions, although the bites of Recluse Spiders, which are native to the Americas, have been confirmed to cause tissue necrosis. Generally, neurotoxic venoms kill more quickly than cytotoxic venoms.

Toxins which attack nerves

The main effect of a neurotoxic venom is to block nerve impulses to the muscles, causing cramps and rigidity and also disrupting many of the body's functions. It also overstimulates the production of the neurotransmitters, acetylcholine and norephinephrine, causing paralysis of the entire nervous system. The combined effect causes sudden and severe stress to the entire human body. In extreme cases, this can result in death due to respiratory or circulatory failure. Funnel-web Spider venom - known as atracotoxin - acts directly upon the nervous system in this way.

Toxins which attack the tissue

Necrotic venoms cause skin blisters around the site of the bite, which may lead to ulcers and tissue death - necrosis. Recent studies of confirmed spider bites suggest that, in Australia, these bites do not cause tissue necrosis. These sorts of symptoms are most likely due to other types of clinical conditions.

Antivenoms for spider toxins are produced by injecting horses, goats or rabbits with the spiders' venom. This doesn't harm these animals because they are either given only small venom doses or they have a naturally mild reaction to the venom. Antibody molecules are produced as a result of the reaction of the animal's immune systems to the foreign venom molecules. These are used to make life-saving antivenoms for humans. Molecular research aimed at making synthetic antivenoms is in progress.

First aid - points to remember

• Those at greatest risk, as with any toxin, are the very young or elderly and those with pre-existing cardiovascular disease.
• Suspected funnel-web or mouse spider bites should always be treated as quickly as possible by applying a pressure bandage and immobilising the victim (do not cut the wound or apply a tight tourniquet).
• Bandaging is not necessary for Redback Spider bites. Applying pressure worsens the pain that often comes with Redback bites.
• The application of a cold pack may help if the bite is painful. For most spider bites, no other first aid is necessary.
• Always seek medical attention for any suspected funnel-web, mouse or Redback Spider bite and for any other bite if symptoms develop or persist.
• Catch the spider for positive identification if you can.

Pressure bandages slow down the movement of venom into the bloodstream, which reduces the effect of the nerve toxins in the venom. Pressure bandages should only be used for funnel-web or mouse spider bites. When the spider bites someone, the venom is injected into the tissue under the skin. A pressure bandage slows down the movement of both tissue fluid and blood near the surface. This prevents the venom from rapidly reaching the bloodstream and is very effective treatment as long as the patient is kept still.

1. Wind the bandage firmly around the bitten arm or leg starting from the bite. The bandage should not be so tight that it restricts blood flow.
2. Wrap the entire limb, then apply a splint to prevent movement.
3. Keep the victim as still as possible.
4. Do not remove the bandage.
5. Seek medical attention as soon as possible.

Catching a spider

• Quickly place an empty jar over the spider
• Push a stiff piece of card under the jar to cover its mouth
• Up-end the jar so that the spider falls to the bottom
• Quickly remove the card and securely replace the jar's lid